I am always the jackass showing up at a restaurant wondering why they are closed and then remembering that it's a federal holiday. There I am on New Year's Day, or July 4th (ie, today) smacking myself upside the head for neglecting to remember that restaurant folk need a day off once in awhile. Like I said, I'm a jackass.

Who else?
In a pinch, I've found that Chinese restaurants tend to be open every day of the year. They are the Old Faithful. Usually there are a few burrito places that are open, too. And if you aren't the type to spend the day with beers and BBQ, but you still want something that screams America (other than Neil Diamond, I mean) then you can hit up Sparky's for burgers and milkshakes.

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Zuni, Bar Jules,
SPQR and
A16 are all definitely closed today. And I certainly don't know that because I ate at the first three yesterday within a span of 6.5 hours (side note: I also certainly did not eat at Bar Jules the previous night as well). Sebo is taking the whole weekend off, and will re-open on Tuesday for dinner. And while those are the only ones I *know* will be closed, I'm guessing 92.37% of all the other restaurants in the city will be closed as well, so call before hop in your car or on Muni (or on your bike or camel or whatever). Also, feel free to add any additional closures you know about to the comments section -- I'm taking a cue from my restaurant buddies and taking the rest of the day off.
What else is closed: Banks, federal and state offices, etc. A list from
SFGate.com.